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Sputter deposited LiPON thin films from powder target as electrolyte for thin film battery applications
Thin Solid Films 519 (2011) 3401–3406
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Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Sputter deposited LiPON thin films from powder target as electrolyte for thin film battery applications C.S. Nimisha a,⁎, K. Yellareswar Rao a, G. Venkatesh b, G. Mohan Rao a, N. Munichandraiah b a b
Department of Instrumentation, Indian Institute of Science, Bangalore 560012, India Department of Inorganic and Physical Chemistry Indian Institute of Science, Bangalore 560012, India
a r t i c l e
i n f o
Article history: Received 6 May 2010 Received in revised form 24 December 2010 Accepted 4 January 2011 Keywords: Powder target Solid electrolyte LiPON thin films X-ray photoelectron spectroscopy
a b s t r a c t Lithium phosphorus oxynitride (LiPON) thin films as solid electrolytes were prepared by reactive radio frequency (rf) magnetron sputtering from Li3PO4 powder compact target. High deposition rates and ease of manufacturing powder target compared with conventional ceramic Li3PO4 targets offer flexibility in handling and reduce the cost associated. Rf power density varied from 1.7 Wcm− 2 to 3 Wcm− 2 and N2 flow from 10 to 30 sccm for a fixed substrate to target distance of 4 cm for best ionic conductivity. The surface chemical analysis done by X-ray photoelectron spectroscopy showed incorporation of nitrogen into the film as both triply, Nt and doubly, Nd coordinated form. With increased presence of Nt, ionic conductivity of LiPON was found to be increasing. The electrochemical impedance spectroscopy of LiPON films confirmed an ionic conductivity of 1.1 × 10− 6 Scm− 1 for optimum rf power and N2 flow conditions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, much importance has been given to the research and development of Li-ion batteries, which are promising candidates for micro-power applications [1–4]. Various research groups are involved in the development of different kinds of Li-ion batteries, in which thin film battery (TFB), is a fast growing category. These batteries make use of a thin solid electrolyte layer between cathode and anode sequentially deposited on to a substrate by various deposition processes, matching to modern semiconductor technologies. Thin films deposited by physical vapor deposition techniques (PVD) can greatly improve reaction areas of rechargeable TFB as they provide cleaner and intimate interface between cathode, electrolyte and anode layers. For any lithium based battery, the electrolyte layer permits the repeated and rapid transfer of Li+ ions between the anode and cathode over an expected range of operating conditions. A thin layer of solid electrolyte provides considerable savings in terms of volume and mass of the battery compared to liquid or polymer electrolyte. Widely used solid electrolytes are either glassy (oxides or sulfides) or ceramic type. Glassy or amorphous electrolytes have several advantages compared to crystalline counterparts; such as wide range of selection in composition, isotropic properties, no grain boundaries, easy film formation etc. Altogether solid electrolytes exhibit higher ionic conductivities with single ion conduction, in which only Li+ ions are mobile while the counter anions and other cations form a rigid
⁎ Corresponding author. Fax: +91 8023600135. E-mail address: [email protected] (C.S. Nimisha). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.01.087
network [5]. This effectively suppresses the undesirable side reactions or decomposition of electrolyte due to anionic concentration gradient across the electrolyte. Different solid electrolyte chemistries include Li2S–SiS2 [6], Li2S–SiS2–Li4SiO4 [7], Li2S–SiS2–Li3PO4 [8], Li2S–P2S5 [9], Li2O–Al2O3–TiO2–SiO2–P2O5 [10] etc. Ionic conductivities in the range 10− 3–10− 4 Scm− 1 have been reported from sulfur based and ceramic electrolytes. Sulfur based electrolytes are moisture sensitive and tend to corrode the deposition equipment making it difficult to handle. High temperature annealing requirement for ceramic electrolytes limits their applications when multi-layered stacks made of materials with varying thermal expansion coefficients are used. As an alternative, amorphous lithium phosphorus oxynitride (LiPON) thin films formed by sputtering from Li3PO4 target in pure N2, with a composition of Li3.3 PO3.9 N0.17 and a Li+-ion conductivity of 2 × 10− 6 Scm− 1 at 25 °C [11] has been integrated as an electrolyte layer of TFB [2–4]. Although LiPON has lower ionic conductivity compared to other solid electrolytes, stability with Li anode, better durability, ease of integration with TFB with a matching potential window of 0–5.5 V favor this oxinitride glassy electrolyte. A recent review on fast Li-ion conductors by Knauth provides more insight in to the state-of-the-art knowledge of solid electrolytes [12]. LiPON electrolyte films have been synthesized by different vapor deposition techniques like rf magnetron sputtering [13–16], ion-beam assisted deposition (IBAD) [17], pulsed laser deposition (PLD) [18], and e-beam evaporation [19]. Among rf sputtering, IBAD, PLD and e-beam evaporation, a maximum ionic conductivity of 3.3 × 10− 6 Scm− 1 has been reported from rf sputtered LiPON films [20]. Rf sputtering is preferred over other PVD processes due to its high repeatability for multi-elemental compounds, formation of pin hole free films with good contacts and high particle energy leading to dense layers. The major
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Fig. 1. Schematic illustration of magnetron sputtering system using Li3PO4 powder target for LiPON deposition.
limiting factor of rf magnetron sputtering from Li3PO4 ceramic target is the low deposition rate (typically 2 nm/min) [20] and a non-uniform erosion which eventually leads to non stoichiometry of the multielemental target. Lee et al. [21] for rf sputtered LiPON films and Kim et al. [22] for plasma assisted PVD of LiPON films have reported higher growth rates with an ionic conductivity in the range of 10− 7–10− 9 Scm− 1. Typically lithium phosphate targets are prepared by a series of steps which includes calcinations of Li3PO4 powder, binder addition, drying, sieving and ball milling followed by hot/cold press to increase the density of the target. This disc has to be sintered for long hours to complete the target making process. But after few depositions, β Li3PO4 transforms to polymorphic γ Li3PO4 due to temperature increase in the target which finally leads to cracking of the sintered targets [23]. Considering the material loss associated and cost of production of ceramic sintered targets, sputtering from powder target is a straight forward and cost effective technique. This avoids cracking of brittle targets due to low thermal conductivity. Sputtering from powder target has been reported for multi-component films such as YBCO, CrB–MoSx and CrB–TiC–MoSx with good repeatability [24,25]. In the present study we have combined the merits of rf sputter deposition and a cost effective powder compact Li3PO4 target in N2 plasma. The goals of this study are to,
Fig. 3. XRD patterns of pure Li3PO4 powder target and LiPON film deposited. (Deposition conditions: rf power density of 3 Wcm− 2, N2 flow of 30 sccm).
2. Experimental details Stoichiometric Li3PO4 powder targets were made by filling Li3PO4 powder (99.99% Sigma Aldrich ) in a 3 inch diameter Cu disk with a trench of 3 mm depth and was packed tightly by pressing with a flat metal plate. This powder target was fixed on to rf magnetron in a sputter-up configuration. The schematic representation of the deposition system with powder target arrangement is shown in Fig. 1.
i. Explain the synthesis of LiPON films in N2 plasma from a powder compact Li3PO4 target with a high deposition rate. ii. Characterize the properties of resulting films in terms of structure, surface morphology, N2 incorporation by elemental composition study and Li+-ion conductivity. iii. Optimize the deposition conditions in terms of rf power and N2 flow during deposition.
Fig. 2. Schematic layout of metal–insulator–metal (MIM) structure of Pt/LiPON/Al for impedance measurement.
Fig. 4. (a) SEM image of LiPON film surface on Pt coated Si substrate and (b) crosssection of Pt/LiPON/Al (MIM) structure. (Deposition conditions are, Rf power density of 3 Wcm− 2, N2 flow of 30 sccm.)
C.S. Nimisha et al. / Thin Solid Films 519 (2011) 3401–3406
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Fig. 5. Impedance spectra of LiPON thin films deposited from powder target at different rf power densities (a) 1.7 Wcm− 2 (b) 2.2 Wcm− 2, (c) 2.6 Wcm− 2, (d) 3 Wcm− 2 from Pt/ LiPON/Al sandwich structure.
Pre-cleaned platinum deposited silicon substrates (Silicon Valley Inc.USA) were held at a distance of 4 cm away from the target. The substrate holder was provided with a to and fro movement above the target for homogeneous deposition of the film over the substrate. A
base vacuum of 1 × 10− 6 mbar was obtained with a turbo molecular pump backed by a rotary pump. The powder compact target was presputtered for an hour to remove any hydrocarbons present and also to sinter the top layers of target locally. Substrate temperature was not controlled during deposition, but was observed to rise up to 110 °C towards the end of deposition due to plasma heating. The deposition time of 40 min was kept constant for all the depositions used in this
Fig. 6. Arrhenius plot of ionic conductivity of LiPON thin film vs. temperature. (Deposition conditions: rf power density of 3 Wcm− 2, nitrogen flow of 30 sccm).
Fig. 7. XPS survey spectra of LiPON film and Li3PO4 powder used as target. (Deposition conditions are, rf power density of 3 Wcm− 2, N2 flow of 30 sccm).
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1.6 V. The AC-impedance measurements were performed at room temperature and at higher temperature (27 °C to 130 °C) using a BioLogic SA potentiostat/Galvanostat (model: VPM3) at frequencies from 1 Hz to 100 KHz. 3. Results and discussion X-ray diffraction patterns of pure Li3PO4 powder and LiPON film deposited on silicon wafer are shown in Fig. 3. While Li3PO4 powder has various crystalline phases, LiPON film did not exhibit any peaks, indicating amorphous nature of the film. This is advantageous for battery applications since the ionic conductivity of amorphous films is generally more isotropic and higher than that of single crystal or textured polycrystalline films [26]. Fig. 4(a) shows the surface micrograph of as deposited LiPON film on Pt coated silicon and the cross-sectional view of Pt/LiPON/Al sandwich structure in Fig. 4(b). The as deposited LiPON film is smooth without any cracks or pin holes on the surface. Also at the interface it makes a clean contact with the top and bottom metal layers, minimizing interfacial resistance between the layers. Li+-ion conductivity of LiPON films was measured by electrochemical impedance spectroscopy. Complex impedance of each of the test pads was measured in 1–105 Hz frequency range at room temperature. The impedance obtained is a characteristic of a singlephase ionic conductor with blocking electrode configuration. The ionic conductivity was calculated from the electrolyte resistance Rel (which is the real part of impedance ZRe value at selected frequency in which −Zim goes through a local minimum-Yu method) using the relation, σ = 1 = Rel × d = A
Fig. 8. (a) Core level spectra of P 2p region showing the shift in binding energy (B.E.) of P 2p for film and Li3PO4 powder used as target, (b) C 1s region of LiPON film and Li3PO4 powder.
study. Rf power density was varied from 1.7 Wcm− 2 to 3 Wcm− 2 and N2 flow from 10 to 40 sccm in order to optimize the processing conditions. The crystal structure of Li3PO4 powder and films was characterized by X-ray diffraction (XRD) using Bruker D8 Advance (Cu-Kα radiation, λ = 1.5405 A0). A SIRION 200 field emission scanning electron microscope (FESEM) was employed to investigate the surface morphology and cross sectional microstructure of LiPON films. All the samples were coated with a thin Pd/Au layer to reduce surface charging effects. X-ray photoelectron spectroscopy (XPS) analysis was performed with SPECS GmbH spectrometer (Phoibos 100 MCD Energy Analyzer) using MgKα radiation (1253.6 eV). The residual pressure inside the analysis chamber was in 10− 10 mbar range. The spectrometer was calibrated using photoemission lines of Ag (Ag 3d3/2 = 367 eV with reference to Fermi level). Peaks were recorded with constant pass energy of 40 eV. Because of surface charge induced peak shifts, C 1s at 284.6 eV was taken as a reference energy position to correct the shift. The N 1s peaks were resolved using a peak synthesis program in which a non-linear background was assumed. The synthetic peaks were defined with a combination of Gaussian and Lorentzian distributions with a fixed FWHM of 1.8 eV. The LiPON film thickness was determined from Dektak 150 stylus profilometer. Ionic conductivity of the films was obtained from the AC-impedance measurement of Pt/LiPON/Al sandwich structures (Fig. 2) fabricated on Pt coated silicon substrates with a LiPON thickness of 1 μm and Al top layer deposited by thermal evaporation to an area of 2 × 2 mm. The test cells showed an open circuit voltage of
where d is the thickness and A is the surface area of contact of LiPON thin films [27]. There have been contradictory reports on the effect of rf power density on ionic conductivity of LiPON film. Earlier Choi et al. [28] reported that ionic conductivity of sputter deposited LiPON films was inversely proportional to rf power density, whereas, studies by Roh et al. supported a directly proportional relation of ionic conductivity with rf power density [29]. Our experiments showed an increased ionic conductivity with increase in rf power density in conformity with the study of Roh et al. The bode plot representation of ionic conductivity obtained from LiPON films deposited with different rf powers is shown in Fig. 5(a–d). It can be seen that as the rf power density is increased from 1.7 Wcm− 2 to 3 Wcm− 2, there is an increase in the ionic conductivity from 2.3 × 10− 9 Scm− 1 to 1.1 × 10− 6 Scm− 1. According to linear fit of Arrhenius equation, impedance analysis showed an increased ionic conductivity with increase in measurement temperature (Fig. 6). The activation energy Ea of LiPON has been calculated using the equation, lnðσT Þ = lnðσ0 T Þ−Ea = kT where Ea is the activation energy, σ is ionic conductivity, T is temperature in Kelvin and k is Boltzmann constant. Activation energy was found to be 0.44 eV for the LiPON film deposited at optimized conditions. In some of the reported studies on LiPON thin films, the rate of deposition was very low from the ceramic target and hence long hours of deposition were employed [20,30]. But we have observed that films of 1.2 μm thick can be deposited from powder target of Li3PO4 in N2 plasma with a deposition time of 40 min. Here the rate of deposition is around 30 nm/min, which is 15 times higher than the sputtering rate from ceramic Li3PO4 target. In order to remove an atom from a target surface, energy greater than surface binding energy (Esurf) has to be supplied. Since for the powder target, atoms are loosely confined to surrounding atoms, the energy required to remove it from the lattice
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Fig. 9. N 1s XPS spectra with component analysis showing triply coordinated nitrogen as Nt and doubly coordinated nitrogen as Nd of LiPON films deposited with 4 different rf powder densities, (a) 1.7 Wcm− 2, (b) 2.2 Wcm− 2, (c) 2.6 Wcm− 2, (d) 3 Wcm− 2.The solid symbols represent the raw data and smooth curve represents the fitted data.
site is less compared with sintered target, which is relatively tight packed. Thus, the higher deposition rate obtained from the powder target compared to a ceramic target can be due to less binding energy associated with the powder target. Also the microscopic unevenness of top surface layer leads to higher effective surface area of powder particles which contributes to high deposition rates.
To investigate the chemical nature of deposited films, XPS analysis of LiPON thin films deposited from powder target was done to provide information about elemental bonding environment. Estimated error in the calculated chemical composition can be around 10% for multielemental compounds [31]. Survey scans of Li3PO4 powder and LiPON films done from 1100 to 10 eV clearly depict the incorporation of
Fig. 10. Ionic conductivity and Nt/Nd ratio of LiPON thin films as a function of rf power density.
Fig. 11. Ionic conductivity and Nt/Nd ratio of LiPON thin films as a function of nitrogen flow.
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nitrogen in to LiPON film, which was otherwise absent in powder sample (Fig. 7). Studies with XPS showed a P 2p peak shift of Li3PO4 from 134.5 to 132.8 eV for LiPON films due to nitrogen incorporation [32]. This reduction in binding energy is attributed to the replacement of P–O bonds by P–N bonds which change the charge distribution around phosphorus in thin films. Core level spectra of P 2p for both film and powder samples are shown in Fig. 8(a) for comparison. Fig. 8(b) shows the corresponding C 1s peaks from both Li3PO4 powder and LiPON film. Also the study of amorphous phosphorous nitrides by Veprek et al. suggested nitrogen incorporation as doubly (–N=) and triply coordinated (–N≤) state [33]. Our studies on LiPON films, revealed nitrogen incorporation in to Li3PO4 as both doubly coordinated ‘Nd’ (peak at, 399.4 eV), or triply coordinated ‘Nt’, (peak at, 400.8 eV) manner. Resolving N 1s spectrum of LiPON films deposited with different rf power densities, into two components and measuring the area of Nt and Nd gives a quantitative measure of each. Fig. 9(a–d) shows N 1s spectrum and its component analysis for the films deposited with different rf power densities of 1.7, 2.2, 2.6 and 3 Wcm− 2. A clear trend on increase in Nt is observed with increase in rf power. The effect of increased Nt/Nd ratio on ionic conductivity of LiPON film with the increase in rf power density is plotted in Fig. 10. Due to the higher ionic radius of N3− compared with O2−, the nitrogen substitution in LiPON film for oxygen in Li3PO4 induces structural distortions, which in fact improves the ionic conductivity and stability of LiPON. The reduction in electrostatic energy, once the P–O bond is replaced by a more covalent P–N bond, lowers the activation energy of Li± mobility in the defect lattice. It was suggested that more structural distortion induced by cross linked Nt than Nd in the LiPON films, consequently improving ionic conductivity with increased triply coordinated nitrogen Nt [29,34]. For an rf power density of 1.7 Wcm− 2, Nt/Nd ratio obtained was 0.27 with an ionic conductivity of 2.3 × 10− 9 Scm− 1. But as the rf power density increased to 3 Wcm− 2, Nt/Nd ratio improved to 1.42 and the ionic conductivity also increased to 1.1× 10− 6 Scm− 1. However increase of rf power density beyond 3 Wcm− 2 was difficult as the powder particles of the target begin to splash to growing thin film surface. Other than the rf power density, nitrogen flow rate is an important process parameter that governs ionic conductivity of LiPON films [18]. We have selected four different flow rates of 10, 20, 30 and 40 sccm of nitrogen to fine tune the conductivity obtained with rf power density of 3 Wcm− 2. The dependence of ionic conductivity on N2 flow rate and Nt / Nd ratio obtained from XPS analysis is shown in Fig. 11. Initially ionic conductivity increases from 7.2 × 10− 9 Scm− 1 to 1.1 × 10− 6 Scm− 1 for a flow rate increase of 10 to 30 sccm. But for 40 sccm of N2, the deposition rate itself reduces due to increased scattering of sputtered species and conductivity reduces to 4.7 × 10− 7 Scm− 1. With the increase in the nitrogen flow, the incorporation of nitrogen into the lithium phosphate matrix in the film as well as the source also increases. Further addition of nitrogen into the source results in a reduction in the sputtering rate. It was observed that the deposition rate was increased from15 nm/min to 30 nm/min as the nitrogen flow increased from 10 to 30 sccm. At a flow rate of 40 sccm, the deposition rate reduced to 20 nm/ min and this effect was seen in the incorporation of nitrogen in the film. It has been also confirmed by XPS studies that a film deposited with 40 sccm of nitrogen flow has lesser Nt/Nd ratio. The maximum obtained ionic conductivity from this study is 1.1 × 10− 6 Scm− 1, which is low compared to the best reported ionic conductivity (3.3 × 10− 6 Scm− 1 of LiPON films by Hu et al. [20]). However, LiPON films with an ionic conductivity of 4.5 × 10− 7 Scm− 1 has already been demonstrated as solid electrolyte of a LiCoO2/LiPON/Li TFB with a discharge capacity of 59 μAh cm− 2μm− 1 and good capacity retention [35].
4. Conclusions In this study we have shown that, sputtering from powder target can be useful for certain compounds like Li3PO4 in which breaking of ceramic target and material loss are severe problems. The ionic conductivity of LiPON films formed was in relative good agreement with previously reported values with a higher deposition rate. The effects of rf power change and N2 flow during deposition are studied in detail. It is seen that with increase in rf power density, ionic conductivity is increased and for increased nitrogen flow, there is an increased ionic conductivity for 10 to 30 sccm but reduces for higher N2 flow of 40 sccm. Amorphous nature of the films during deposition process has been ensured and verified through XRD. Incorporation of triply coordinated nitrogen enhances the ionic conductivity which has been confirmed by XPS and AC impedance analysis. A maximum ionic conductivity of 1.1 × 10− 6 Scm− 1 was obtained for an Nt/Nd ratio of 1.42 for 3 Wcm− 2 and N2 flow of 30 sccm. Acknowledgement The authors acknowledge DRDO, Govt. of India for funding this work. References [1] J.W. Schultze, T. Osaka, M. Datta (Eds.), Electrochemical Microsystem Technologies, CRC Press, 2002. [2] B.J. Neudecker, N.J. Dudney, J.B. Bates, J. Electrochem. Soc. 147 (2000) 517. [3] H.J. Ji, S.H. Kang, H.J. Lee, P.H. Kim, S.B. Cho, Proc. IME. G J. Aero. Eng. 223 (2009) 107. [4] W.Y. Liu, Z.W. Fu, Q.Z. Qin, J. Electrochem. Soc. 155 (2008) A8. [5] T. Minami, Solid State Ionics for Batteries, Springer-Verlag, Tokyo, 2005. [6] H. Morimoto, H. Yamashita, M. Tatsumisago, T. Minami, J. Am. Ceram. Soc. 82 (1999) 1352. [7] H. Morimoto, H. Yamashita, M. Tatsumisago, T. Minami, J. Ceram. Soc. Jpn., Int. Ed. 108 (2000) 128. [8] M. Tatsumisago, H. Yamashita, A. Hayashi, H. Morimoto, T. Minami, J. Non-Cryst. Solids 274 (2000) 30. [9] F. Mizuno, A. Hayashi, K. Tadanaga, M. Tatsumisago, Solid State Ionics 177 (2006) 2721. [10] J. Fu, J. Am. Ceram. Soc. 80 (1997) 1901. [11] J.B. Bates, N.J. Dudney, G.R. Gruzalski, R.A. Zuhr, A. Choudhury, C.F. Luck, Solid State Ionics 53–56 (1992) 647. [12] P. Knauth, Solid State Ionics 180 (2009) 911. [13] Y. Hamon, A. Douard, F. Sabary, C. Marcel, P. Vinatier, B. Pecquenard, A. Levasseur, Solid State Ionics 177 (2006) 257. [14] N.J. Dudney, J.B. Bates, R.A. Zuhr, C.F. Luck, Solid State Ionics 53–56 (1992) 655. [15] W.C. West, J.F. Whitacre, J.R. Lim, J. Power Sources 126 (2004) 134. [16] J. Schwenzel, V. Thangadurai, W. Weppner, J. Power Sources 154 (2006) 232. [17] F. Vereda, R.B. Goldner, T.E. Haas, P. Zerigian, Electrochem. Solid-State Lett. 5 (2002) A239. [18] S. Zhao, Z. Fu, Q. Qin, Thin Solid Films 415 (2002) 108. [19] W.Y. Liu, Z.W. Fu, C.L. Li, Q.Z. Qin, Electrochem. Solid-State Lett. 7 (2004) J36. [20] Z. Hu, D. Li, K. Xie, Bull. Mater. Sci. 31 (2008) 681. [21] J.M. Lee, S.H. Kim, Y. Tak, J.M. Yoon, J. Power Sources 163 (2006) 173. [22] Y.G. Kim, H.N.G. Wadley, J. Vac. Sci. Technol., A 26–1 (2008) 174. [23] B. Wang, B.C. Chakoumakos, B.C. Sales, B.S. Kwak, J.B. Bates, J. Solid State Chem. 115 (1995) 313. [24] W.G. Luo, A.L. Ding, K.S. Chan, G.G. Siu, A. Cheng, E.C.M. Young, J. Supercond. 5 (1992) 239. [25] M. Audronis, P.J. Kelly, R.D. Arnell, A. Leyland, A. Matthews, Surf. Coat. Technol. 200 (2005) 1616. [26] P.G. Bruce, Solid State Electrochemistry, Cambridge University Press, 1997. [27] X. Yu, J.B. Bates, G.E. Jellison Jr., F.X. Hart, J. Electrochem. Soc. 144 (1997) 524. [28] C.H. Choi, W.I. Cho, B.W. Cho, H.S. Kim, Y.S. Yoon, Y.S. Taka, Electrochem. SolidState Lett. 5 (2002) A14. [29] N.S. Roh, S.D. Lee, H.S. Kwon, Scr. Mater. 42 (2000) 43. [30] N.J. Dudney, J.B. Bates, J.D. Robertson, J. Vac. Sci. Technol., A 11 (1993) 377. [31] D. Brigs, M.P. Seah (Eds.), Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, Mir, Moscow, 1987. [32] B.K. Brow, C.G. Pantano, J. Am. Ceram. Soc. 69 (1986) 314. [33] S. Veprek, S. Iqbal, J. Brunner, M. Scharli, Philos. Mag. 43 (1981) 527. [34] J.B. Bates, N.J. Dudney, G.R. Gruzalski, R.A. Zuhr, A. Choudhury, C.F. Luck, J.D. Robertson, J. Power Sources 43–44 (1993) 103. [35] N. Kuwata, N. Iwagami, Y. Matsuda, Y. Tanji, J. Kawamura, ECS Trans. 16 /26 (2009) 53.
Contents lists available at ScienceDirect
Thin Solid Films j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / t s f
Sputter deposited LiPON thin films from powder target as electrolyte for thin film battery applications C.S. Nimisha a,⁎, K. Yellareswar Rao a, G. Venkatesh b, G. Mohan Rao a, N. Munichandraiah b a b
Department of Instrumentation, Indian Institute of Science, Bangalore 560012, India Department of Inorganic and Physical Chemistry Indian Institute of Science, Bangalore 560012, India
a r t i c l e
i n f o
Article history: Received 6 May 2010 Received in revised form 24 December 2010 Accepted 4 January 2011 Keywords: Powder target Solid electrolyte LiPON thin films X-ray photoelectron spectroscopy
a b s t r a c t Lithium phosphorus oxynitride (LiPON) thin films as solid electrolytes were prepared by reactive radio frequency (rf) magnetron sputtering from Li3PO4 powder compact target. High deposition rates and ease of manufacturing powder target compared with conventional ceramic Li3PO4 targets offer flexibility in handling and reduce the cost associated. Rf power density varied from 1.7 Wcm− 2 to 3 Wcm− 2 and N2 flow from 10 to 30 sccm for a fixed substrate to target distance of 4 cm for best ionic conductivity. The surface chemical analysis done by X-ray photoelectron spectroscopy showed incorporation of nitrogen into the film as both triply, Nt and doubly, Nd coordinated form. With increased presence of Nt, ionic conductivity of LiPON was found to be increasing. The electrochemical impedance spectroscopy of LiPON films confirmed an ionic conductivity of 1.1 × 10− 6 Scm− 1 for optimum rf power and N2 flow conditions. © 2011 Elsevier B.V. All rights reserved.
1. Introduction In recent years, much importance has been given to the research and development of Li-ion batteries, which are promising candidates for micro-power applications [1–4]. Various research groups are involved in the development of different kinds of Li-ion batteries, in which thin film battery (TFB), is a fast growing category. These batteries make use of a thin solid electrolyte layer between cathode and anode sequentially deposited on to a substrate by various deposition processes, matching to modern semiconductor technologies. Thin films deposited by physical vapor deposition techniques (PVD) can greatly improve reaction areas of rechargeable TFB as they provide cleaner and intimate interface between cathode, electrolyte and anode layers. For any lithium based battery, the electrolyte layer permits the repeated and rapid transfer of Li+ ions between the anode and cathode over an expected range of operating conditions. A thin layer of solid electrolyte provides considerable savings in terms of volume and mass of the battery compared to liquid or polymer electrolyte. Widely used solid electrolytes are either glassy (oxides or sulfides) or ceramic type. Glassy or amorphous electrolytes have several advantages compared to crystalline counterparts; such as wide range of selection in composition, isotropic properties, no grain boundaries, easy film formation etc. Altogether solid electrolytes exhibit higher ionic conductivities with single ion conduction, in which only Li+ ions are mobile while the counter anions and other cations form a rigid
⁎ Corresponding author. Fax: +91 8023600135. E-mail address: [email protected] (C.S. Nimisha). 0040-6090/$ – see front matter © 2011 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2011.01.087
network [5]. This effectively suppresses the undesirable side reactions or decomposition of electrolyte due to anionic concentration gradient across the electrolyte. Different solid electrolyte chemistries include Li2S–SiS2 [6], Li2S–SiS2–Li4SiO4 [7], Li2S–SiS2–Li3PO4 [8], Li2S–P2S5 [9], Li2O–Al2O3–TiO2–SiO2–P2O5 [10] etc. Ionic conductivities in the range 10− 3–10− 4 Scm− 1 have been reported from sulfur based and ceramic electrolytes. Sulfur based electrolytes are moisture sensitive and tend to corrode the deposition equipment making it difficult to handle. High temperature annealing requirement for ceramic electrolytes limits their applications when multi-layered stacks made of materials with varying thermal expansion coefficients are used. As an alternative, amorphous lithium phosphorus oxynitride (LiPON) thin films formed by sputtering from Li3PO4 target in pure N2, with a composition of Li3.3 PO3.9 N0.17 and a Li+-ion conductivity of 2 × 10− 6 Scm− 1 at 25 °C [11] has been integrated as an electrolyte layer of TFB [2–4]. Although LiPON has lower ionic conductivity compared to other solid electrolytes, stability with Li anode, better durability, ease of integration with TFB with a matching potential window of 0–5.5 V favor this oxinitride glassy electrolyte. A recent review on fast Li-ion conductors by Knauth provides more insight in to the state-of-the-art knowledge of solid electrolytes [12]. LiPON electrolyte films have been synthesized by different vapor deposition techniques like rf magnetron sputtering [13–16], ion-beam assisted deposition (IBAD) [17], pulsed laser deposition (PLD) [18], and e-beam evaporation [19]. Among rf sputtering, IBAD, PLD and e-beam evaporation, a maximum ionic conductivity of 3.3 × 10− 6 Scm− 1 has been reported from rf sputtered LiPON films [20]. Rf sputtering is preferred over other PVD processes due to its high repeatability for multi-elemental compounds, formation of pin hole free films with good contacts and high particle energy leading to dense layers. The major
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Fig. 1. Schematic illustration of magnetron sputtering system using Li3PO4 powder target for LiPON deposition.
limiting factor of rf magnetron sputtering from Li3PO4 ceramic target is the low deposition rate (typically 2 nm/min) [20] and a non-uniform erosion which eventually leads to non stoichiometry of the multielemental target. Lee et al. [21] for rf sputtered LiPON films and Kim et al. [22] for plasma assisted PVD of LiPON films have reported higher growth rates with an ionic conductivity in the range of 10− 7–10− 9 Scm− 1. Typically lithium phosphate targets are prepared by a series of steps which includes calcinations of Li3PO4 powder, binder addition, drying, sieving and ball milling followed by hot/cold press to increase the density of the target. This disc has to be sintered for long hours to complete the target making process. But after few depositions, β Li3PO4 transforms to polymorphic γ Li3PO4 due to temperature increase in the target which finally leads to cracking of the sintered targets [23]. Considering the material loss associated and cost of production of ceramic sintered targets, sputtering from powder target is a straight forward and cost effective technique. This avoids cracking of brittle targets due to low thermal conductivity. Sputtering from powder target has been reported for multi-component films such as YBCO, CrB–MoSx and CrB–TiC–MoSx with good repeatability [24,25]. In the present study we have combined the merits of rf sputter deposition and a cost effective powder compact Li3PO4 target in N2 plasma. The goals of this study are to,
Fig. 3. XRD patterns of pure Li3PO4 powder target and LiPON film deposited. (Deposition conditions: rf power density of 3 Wcm− 2, N2 flow of 30 sccm).
2. Experimental details Stoichiometric Li3PO4 powder targets were made by filling Li3PO4 powder (99.99% Sigma Aldrich ) in a 3 inch diameter Cu disk with a trench of 3 mm depth and was packed tightly by pressing with a flat metal plate. This powder target was fixed on to rf magnetron in a sputter-up configuration. The schematic representation of the deposition system with powder target arrangement is shown in Fig. 1.
i. Explain the synthesis of LiPON films in N2 plasma from a powder compact Li3PO4 target with a high deposition rate. ii. Characterize the properties of resulting films in terms of structure, surface morphology, N2 incorporation by elemental composition study and Li+-ion conductivity. iii. Optimize the deposition conditions in terms of rf power and N2 flow during deposition.
Fig. 2. Schematic layout of metal–insulator–metal (MIM) structure of Pt/LiPON/Al for impedance measurement.
Fig. 4. (a) SEM image of LiPON film surface on Pt coated Si substrate and (b) crosssection of Pt/LiPON/Al (MIM) structure. (Deposition conditions are, Rf power density of 3 Wcm− 2, N2 flow of 30 sccm.)
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Fig. 5. Impedance spectra of LiPON thin films deposited from powder target at different rf power densities (a) 1.7 Wcm− 2 (b) 2.2 Wcm− 2, (c) 2.6 Wcm− 2, (d) 3 Wcm− 2 from Pt/ LiPON/Al sandwich structure.
Pre-cleaned platinum deposited silicon substrates (Silicon Valley Inc.USA) were held at a distance of 4 cm away from the target. The substrate holder was provided with a to and fro movement above the target for homogeneous deposition of the film over the substrate. A
base vacuum of 1 × 10− 6 mbar was obtained with a turbo molecular pump backed by a rotary pump. The powder compact target was presputtered for an hour to remove any hydrocarbons present and also to sinter the top layers of target locally. Substrate temperature was not controlled during deposition, but was observed to rise up to 110 °C towards the end of deposition due to plasma heating. The deposition time of 40 min was kept constant for all the depositions used in this
Fig. 6. Arrhenius plot of ionic conductivity of LiPON thin film vs. temperature. (Deposition conditions: rf power density of 3 Wcm− 2, nitrogen flow of 30 sccm).
Fig. 7. XPS survey spectra of LiPON film and Li3PO4 powder used as target. (Deposition conditions are, rf power density of 3 Wcm− 2, N2 flow of 30 sccm).
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1.6 V. The AC-impedance measurements were performed at room temperature and at higher temperature (27 °C to 130 °C) using a BioLogic SA potentiostat/Galvanostat (model: VPM3) at frequencies from 1 Hz to 100 KHz. 3. Results and discussion X-ray diffraction patterns of pure Li3PO4 powder and LiPON film deposited on silicon wafer are shown in Fig. 3. While Li3PO4 powder has various crystalline phases, LiPON film did not exhibit any peaks, indicating amorphous nature of the film. This is advantageous for battery applications since the ionic conductivity of amorphous films is generally more isotropic and higher than that of single crystal or textured polycrystalline films [26]. Fig. 4(a) shows the surface micrograph of as deposited LiPON film on Pt coated silicon and the cross-sectional view of Pt/LiPON/Al sandwich structure in Fig. 4(b). The as deposited LiPON film is smooth without any cracks or pin holes on the surface. Also at the interface it makes a clean contact with the top and bottom metal layers, minimizing interfacial resistance between the layers. Li+-ion conductivity of LiPON films was measured by electrochemical impedance spectroscopy. Complex impedance of each of the test pads was measured in 1–105 Hz frequency range at room temperature. The impedance obtained is a characteristic of a singlephase ionic conductor with blocking electrode configuration. The ionic conductivity was calculated from the electrolyte resistance Rel (which is the real part of impedance ZRe value at selected frequency in which −Zim goes through a local minimum-Yu method) using the relation, σ = 1 = Rel × d = A
Fig. 8. (a) Core level spectra of P 2p region showing the shift in binding energy (B.E.) of P 2p for film and Li3PO4 powder used as target, (b) C 1s region of LiPON film and Li3PO4 powder.
study. Rf power density was varied from 1.7 Wcm− 2 to 3 Wcm− 2 and N2 flow from 10 to 40 sccm in order to optimize the processing conditions. The crystal structure of Li3PO4 powder and films was characterized by X-ray diffraction (XRD) using Bruker D8 Advance (Cu-Kα radiation, λ = 1.5405 A0). A SIRION 200 field emission scanning electron microscope (FESEM) was employed to investigate the surface morphology and cross sectional microstructure of LiPON films. All the samples were coated with a thin Pd/Au layer to reduce surface charging effects. X-ray photoelectron spectroscopy (XPS) analysis was performed with SPECS GmbH spectrometer (Phoibos 100 MCD Energy Analyzer) using MgKα radiation (1253.6 eV). The residual pressure inside the analysis chamber was in 10− 10 mbar range. The spectrometer was calibrated using photoemission lines of Ag (Ag 3d3/2 = 367 eV with reference to Fermi level). Peaks were recorded with constant pass energy of 40 eV. Because of surface charge induced peak shifts, C 1s at 284.6 eV was taken as a reference energy position to correct the shift. The N 1s peaks were resolved using a peak synthesis program in which a non-linear background was assumed. The synthetic peaks were defined with a combination of Gaussian and Lorentzian distributions with a fixed FWHM of 1.8 eV. The LiPON film thickness was determined from Dektak 150 stylus profilometer. Ionic conductivity of the films was obtained from the AC-impedance measurement of Pt/LiPON/Al sandwich structures (Fig. 2) fabricated on Pt coated silicon substrates with a LiPON thickness of 1 μm and Al top layer deposited by thermal evaporation to an area of 2 × 2 mm. The test cells showed an open circuit voltage of
where d is the thickness and A is the surface area of contact of LiPON thin films [27]. There have been contradictory reports on the effect of rf power density on ionic conductivity of LiPON film. Earlier Choi et al. [28] reported that ionic conductivity of sputter deposited LiPON films was inversely proportional to rf power density, whereas, studies by Roh et al. supported a directly proportional relation of ionic conductivity with rf power density [29]. Our experiments showed an increased ionic conductivity with increase in rf power density in conformity with the study of Roh et al. The bode plot representation of ionic conductivity obtained from LiPON films deposited with different rf powers is shown in Fig. 5(a–d). It can be seen that as the rf power density is increased from 1.7 Wcm− 2 to 3 Wcm− 2, there is an increase in the ionic conductivity from 2.3 × 10− 9 Scm− 1 to 1.1 × 10− 6 Scm− 1. According to linear fit of Arrhenius equation, impedance analysis showed an increased ionic conductivity with increase in measurement temperature (Fig. 6). The activation energy Ea of LiPON has been calculated using the equation, lnðσT Þ = lnðσ0 T Þ−Ea = kT where Ea is the activation energy, σ is ionic conductivity, T is temperature in Kelvin and k is Boltzmann constant. Activation energy was found to be 0.44 eV for the LiPON film deposited at optimized conditions. In some of the reported studies on LiPON thin films, the rate of deposition was very low from the ceramic target and hence long hours of deposition were employed [20,30]. But we have observed that films of 1.2 μm thick can be deposited from powder target of Li3PO4 in N2 plasma with a deposition time of 40 min. Here the rate of deposition is around 30 nm/min, which is 15 times higher than the sputtering rate from ceramic Li3PO4 target. In order to remove an atom from a target surface, energy greater than surface binding energy (Esurf) has to be supplied. Since for the powder target, atoms are loosely confined to surrounding atoms, the energy required to remove it from the lattice
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Fig. 9. N 1s XPS spectra with component analysis showing triply coordinated nitrogen as Nt and doubly coordinated nitrogen as Nd of LiPON films deposited with 4 different rf powder densities, (a) 1.7 Wcm− 2, (b) 2.2 Wcm− 2, (c) 2.6 Wcm− 2, (d) 3 Wcm− 2.The solid symbols represent the raw data and smooth curve represents the fitted data.
site is less compared with sintered target, which is relatively tight packed. Thus, the higher deposition rate obtained from the powder target compared to a ceramic target can be due to less binding energy associated with the powder target. Also the microscopic unevenness of top surface layer leads to higher effective surface area of powder particles which contributes to high deposition rates.
To investigate the chemical nature of deposited films, XPS analysis of LiPON thin films deposited from powder target was done to provide information about elemental bonding environment. Estimated error in the calculated chemical composition can be around 10% for multielemental compounds [31]. Survey scans of Li3PO4 powder and LiPON films done from 1100 to 10 eV clearly depict the incorporation of
Fig. 10. Ionic conductivity and Nt/Nd ratio of LiPON thin films as a function of rf power density.
Fig. 11. Ionic conductivity and Nt/Nd ratio of LiPON thin films as a function of nitrogen flow.
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nitrogen in to LiPON film, which was otherwise absent in powder sample (Fig. 7). Studies with XPS showed a P 2p peak shift of Li3PO4 from 134.5 to 132.8 eV for LiPON films due to nitrogen incorporation [32]. This reduction in binding energy is attributed to the replacement of P–O bonds by P–N bonds which change the charge distribution around phosphorus in thin films. Core level spectra of P 2p for both film and powder samples are shown in Fig. 8(a) for comparison. Fig. 8(b) shows the corresponding C 1s peaks from both Li3PO4 powder and LiPON film. Also the study of amorphous phosphorous nitrides by Veprek et al. suggested nitrogen incorporation as doubly (–N=) and triply coordinated (–N≤) state [33]. Our studies on LiPON films, revealed nitrogen incorporation in to Li3PO4 as both doubly coordinated ‘Nd’ (peak at, 399.4 eV), or triply coordinated ‘Nt’, (peak at, 400.8 eV) manner. Resolving N 1s spectrum of LiPON films deposited with different rf power densities, into two components and measuring the area of Nt and Nd gives a quantitative measure of each. Fig. 9(a–d) shows N 1s spectrum and its component analysis for the films deposited with different rf power densities of 1.7, 2.2, 2.6 and 3 Wcm− 2. A clear trend on increase in Nt is observed with increase in rf power. The effect of increased Nt/Nd ratio on ionic conductivity of LiPON film with the increase in rf power density is plotted in Fig. 10. Due to the higher ionic radius of N3− compared with O2−, the nitrogen substitution in LiPON film for oxygen in Li3PO4 induces structural distortions, which in fact improves the ionic conductivity and stability of LiPON. The reduction in electrostatic energy, once the P–O bond is replaced by a more covalent P–N bond, lowers the activation energy of Li± mobility in the defect lattice. It was suggested that more structural distortion induced by cross linked Nt than Nd in the LiPON films, consequently improving ionic conductivity with increased triply coordinated nitrogen Nt [29,34]. For an rf power density of 1.7 Wcm− 2, Nt/Nd ratio obtained was 0.27 with an ionic conductivity of 2.3 × 10− 9 Scm− 1. But as the rf power density increased to 3 Wcm− 2, Nt/Nd ratio improved to 1.42 and the ionic conductivity also increased to 1.1× 10− 6 Scm− 1. However increase of rf power density beyond 3 Wcm− 2 was difficult as the powder particles of the target begin to splash to growing thin film surface. Other than the rf power density, nitrogen flow rate is an important process parameter that governs ionic conductivity of LiPON films [18]. We have selected four different flow rates of 10, 20, 30 and 40 sccm of nitrogen to fine tune the conductivity obtained with rf power density of 3 Wcm− 2. The dependence of ionic conductivity on N2 flow rate and Nt / Nd ratio obtained from XPS analysis is shown in Fig. 11. Initially ionic conductivity increases from 7.2 × 10− 9 Scm− 1 to 1.1 × 10− 6 Scm− 1 for a flow rate increase of 10 to 30 sccm. But for 40 sccm of N2, the deposition rate itself reduces due to increased scattering of sputtered species and conductivity reduces to 4.7 × 10− 7 Scm− 1. With the increase in the nitrogen flow, the incorporation of nitrogen into the lithium phosphate matrix in the film as well as the source also increases. Further addition of nitrogen into the source results in a reduction in the sputtering rate. It was observed that the deposition rate was increased from15 nm/min to 30 nm/min as the nitrogen flow increased from 10 to 30 sccm. At a flow rate of 40 sccm, the deposition rate reduced to 20 nm/ min and this effect was seen in the incorporation of nitrogen in the film. It has been also confirmed by XPS studies that a film deposited with 40 sccm of nitrogen flow has lesser Nt/Nd ratio. The maximum obtained ionic conductivity from this study is 1.1 × 10− 6 Scm− 1, which is low compared to the best reported ionic conductivity (3.3 × 10− 6 Scm− 1 of LiPON films by Hu et al. [20]). However, LiPON films with an ionic conductivity of 4.5 × 10− 7 Scm− 1 has already been demonstrated as solid electrolyte of a LiCoO2/LiPON/Li TFB with a discharge capacity of 59 μAh cm− 2μm− 1 and good capacity retention [35].
4. Conclusions In this study we have shown that, sputtering from powder target can be useful for certain compounds like Li3PO4 in which breaking of ceramic target and material loss are severe problems. The ionic conductivity of LiPON films formed was in relative good agreement with previously reported values with a higher deposition rate. The effects of rf power change and N2 flow during deposition are studied in detail. It is seen that with increase in rf power density, ionic conductivity is increased and for increased nitrogen flow, there is an increased ionic conductivity for 10 to 30 sccm but reduces for higher N2 flow of 40 sccm. Amorphous nature of the films during deposition process has been ensured and verified through XRD. Incorporation of triply coordinated nitrogen enhances the ionic conductivity which has been confirmed by XPS and AC impedance analysis. A maximum ionic conductivity of 1.1 × 10− 6 Scm− 1 was obtained for an Nt/Nd ratio of 1.42 for 3 Wcm− 2 and N2 flow of 30 sccm. Acknowledgement The authors acknowledge DRDO, Govt. of India for funding this work. References [1] J.W. Schultze, T. Osaka, M. Datta (Eds.), Electrochemical Microsystem Technologies, CRC Press, 2002. [2] B.J. Neudecker, N.J. Dudney, J.B. Bates, J. Electrochem. Soc. 147 (2000) 517. [3] H.J. Ji, S.H. Kang, H.J. Lee, P.H. Kim, S.B. Cho, Proc. IME. G J. Aero. Eng. 223 (2009) 107. [4] W.Y. Liu, Z.W. Fu, Q.Z. Qin, J. Electrochem. Soc. 155 (2008) A8. [5] T. Minami, Solid State Ionics for Batteries, Springer-Verlag, Tokyo, 2005. [6] H. Morimoto, H. Yamashita, M. Tatsumisago, T. Minami, J. Am. Ceram. Soc. 82 (1999) 1352. [7] H. Morimoto, H. Yamashita, M. Tatsumisago, T. Minami, J. Ceram. Soc. Jpn., Int. Ed. 108 (2000) 128. [8] M. Tatsumisago, H. Yamashita, A. Hayashi, H. Morimoto, T. Minami, J. Non-Cryst. Solids 274 (2000) 30. [9] F. Mizuno, A. Hayashi, K. Tadanaga, M. Tatsumisago, Solid State Ionics 177 (2006) 2721. [10] J. Fu, J. Am. Ceram. Soc. 80 (1997) 1901. [11] J.B. Bates, N.J. Dudney, G.R. Gruzalski, R.A. Zuhr, A. Choudhury, C.F. Luck, Solid State Ionics 53–56 (1992) 647. [12] P. Knauth, Solid State Ionics 180 (2009) 911. [13] Y. Hamon, A. Douard, F. Sabary, C. Marcel, P. Vinatier, B. Pecquenard, A. Levasseur, Solid State Ionics 177 (2006) 257. [14] N.J. Dudney, J.B. Bates, R.A. Zuhr, C.F. Luck, Solid State Ionics 53–56 (1992) 655. [15] W.C. West, J.F. Whitacre, J.R. Lim, J. Power Sources 126 (2004) 134. [16] J. Schwenzel, V. Thangadurai, W. Weppner, J. Power Sources 154 (2006) 232. [17] F. Vereda, R.B. Goldner, T.E. Haas, P. Zerigian, Electrochem. Solid-State Lett. 5 (2002) A239. [18] S. Zhao, Z. Fu, Q. Qin, Thin Solid Films 415 (2002) 108. [19] W.Y. Liu, Z.W. Fu, C.L. Li, Q.Z. Qin, Electrochem. Solid-State Lett. 7 (2004) J36. [20] Z. Hu, D. Li, K. Xie, Bull. Mater. Sci. 31 (2008) 681. [21] J.M. Lee, S.H. Kim, Y. Tak, J.M. Yoon, J. Power Sources 163 (2006) 173. [22] Y.G. Kim, H.N.G. Wadley, J. Vac. Sci. Technol., A 26–1 (2008) 174. [23] B. Wang, B.C. Chakoumakos, B.C. Sales, B.S. Kwak, J.B. Bates, J. Solid State Chem. 115 (1995) 313. [24] W.G. Luo, A.L. Ding, K.S. Chan, G.G. Siu, A. Cheng, E.C.M. Young, J. Supercond. 5 (1992) 239. [25] M. Audronis, P.J. Kelly, R.D. Arnell, A. Leyland, A. Matthews, Surf. Coat. Technol. 200 (2005) 1616. [26] P.G. Bruce, Solid State Electrochemistry, Cambridge University Press, 1997. [27] X. Yu, J.B. Bates, G.E. Jellison Jr., F.X. Hart, J. Electrochem. Soc. 144 (1997) 524. [28] C.H. Choi, W.I. Cho, B.W. Cho, H.S. Kim, Y.S. Yoon, Y.S. Taka, Electrochem. SolidState Lett. 5 (2002) A14. [29] N.S. Roh, S.D. Lee, H.S. Kwon, Scr. Mater. 42 (2000) 43. [30] N.J. Dudney, J.B. Bates, J.D. Robertson, J. Vac. Sci. Technol., A 11 (1993) 377. [31] D. Brigs, M.P. Seah (Eds.), Practical Surface Analysis by Auger and X-ray Photoelectron Spectroscopy, Mir, Moscow, 1987. [32] B.K. Brow, C.G. Pantano, J. Am. Ceram. Soc. 69 (1986) 314. [33] S. Veprek, S. Iqbal, J. Brunner, M. Scharli, Philos. Mag. 43 (1981) 527. [34] J.B. Bates, N.J. Dudney, G.R. Gruzalski, R.A. Zuhr, A. Choudhury, C.F. Luck, J.D. Robertson, J. Power Sources 43–44 (1993) 103. [35] N. Kuwata, N. Iwagami, Y. Matsuda, Y. Tanji, J. Kawamura, ECS Trans. 16 /26 (2009) 53.